When I bought my 78 LRT in 1994 it was going cheap. I was the 5th person to look and test drive it. It ran like crap.
I paid the $1200 cash and drove it the 12 miles home and it was way down on power. Doing 45 MPH, I lost speed climbing a long hill going home. I parked it for a week. This is when I found out the engine was a 1978 Warranty replacement E58.

I pulled a compression test and all were at about 145 PSI but #3 was way down, like 80 PSI WET.
Took the valve cover off and found the #3 intake rocker's push rod cup was broken out, windowed but still attached. Push rod was OK but the lifter was almost popped out of its bore. Using my 2.2 coil spring compressor I could push the valve down with no binding. Took a spare rocker and put it back together. Ran excellent. Never another issue in the 200,000 miles I drove it